# Zero Acceleration in Running and in a Car

1. Aug 5, 2015

### gamz95

1) While running with a constant velocity ; how do we consume energy? : Only due to vertical movement of our bodies, right?

2) When driving a car in a constant velocity; how this car could consume energy? It does not move upright-downright (in a very straight road).

2. Aug 5, 2015

### 462chevelle

1) If you ignore all of the bodily operations that make your body work, you are consuming energy by muscle contractions. You consume energy by horizontal movement too, you have to push on the earth in the opposite direction as your motion. As a result of Newtons 3rd law, the earth pushes you forward.

2) Whenever a car is moving it has to overcome fluid friction (wind resistance).

I think you are thinking of the work-potential energy concept. Where you only do work in vertical motion by changing your potential energy.

3. Aug 5, 2015

### gamz95

1) Do I push myself into the earth; or the earth pushes me??

2) I understand. There wouldn't be a case that wind is in the direction of a car, right; so that car's velocity increases?

4. Aug 5, 2015

### Drakkith

Staff Emeritus
Both. There is a force pair between yourself and the Earth. However hard you press down on the Earth, the Earth pushes up against you.

A tailwind would certainly help the car's fuel efficiency, but there's still other sources of friction that will rob the car of energy, such as in the engine, axles, etc. And unless you're going very slow it is very, very unlikely that the tailwind is as fast as the car, so you'll usually still be encountering wind resistance.

5. Aug 5, 2015

### gamz95

Thank you very much people. Thank you!

6. Aug 5, 2015

### CWatters

There are lots of reasons humans consume energy when running. Even air resistance has an effect. It's why records only count if the wind is below a certain speed.

When running your legs behave a bit like springs that store and release energy - however the process isn't very efficient. When disabled athletes started using carbon fibre running blades there was some concern that the blades would be more efficient and make running easier/faster that it is for able bodied athletes. More on that here.. https://en.wikipedia.org/wiki/Mechanics_of_Oscar_Pistorius'_running_blades

In addition to air resistance, cars have lots of moving parts that rub against each other loosing energy to friction. For example pistons in the engine, gears in the transmission etc. Energy is also required to accelerate the mass of the car. That energy is usually lost as heat in the brakes when slowing down.